ABSTRACT Glioblastoma is the most common malignant primary brain tumor in adults and is invariably fatal. Glioblastoma stem-like cells (GSCs) represent a dynamic subpopulation of glioblastoma cells, which are now known to exhibit treatment resistance and cause tumor recurrence. Therefore, knowledge of the mechanisms governing the GSC state is essential to develop effective therapies against glioblastoma. Using patient-derived glioblastoma cell lines, we have recently uncovered a requirement for the E3 ligase CDC20-Anaphase- Promoting Complex (CDC20-APC) in key GSC functions of invasion, self-renewal, and tumor initiation. Furthermore, we have identified the pluripotency regulatory transcription factor SOX2 as a novel CDC20- interacting protein, which mediates the downstream effects of CDC20-APC in GSCs in vitro. How exactly the CDC20-APC/SOX2 signaling pathway is regulated in GSCs and what role the CDC20-APC/SOX2 pathway plays in glioblastoma in vivo remain to be defined. The goals of our proposal are: 1) to identify the molecular mechanisms underlying the CDC20-APC/SOX2 pathway in GSC invasiveness, 2) to determine the impact of the CDC20-APC/SOX2 pathway on tumor initiation and maintenance in glioblastoma in vivo, and 3) to determine the effect of the CDC20-APC/SOX2 pathway on GSC responsiveness to standard-of-care chemotherapy temozolomide and radiation therapy. The long-term goal of this project is to develop novel CDC20-APC-directed therapeutic strategies to disrupt the GSC state and enhance the effectiveness of current treatments.